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Subject[PATCH 4.9 208/271] nvme: retain split access workaround for capability reads
From: Ard Biesheuvel <>

[ Upstream commit 3a8ecc935efabdad106b5e06d07b150c394b4465 ]

Commit 7fd8930f26be4

"nvme: add a common helper to read Identify Controller data"

has re-introduced an issue that we have attempted to work around in the
past, in commit a310acd7a7ea ("NVMe: use split lo_hi_{read,write}q").

The problem is that some PCIe NVMe controllers do not implement 64-bit
outbound accesses correctly, which is why the commit above switched
to using lo_hi_[read|write]q for all 64-bit BAR accesses occuring in
the code.

In the mean time, the NVMe subsystem has been refactored, and now calls
into the PCIe support layer for NVMe via a .reg_read64() method, which
fails to use lo_hi_readq(), and thus reintroduces the problem that the
workaround above aimed to address.

Given that, at the moment, .reg_read64() is only used to read the
capability register [which is known to tolerate split reads], let's
switch .reg_read64() to lo_hi_readq() as well.

This fixes a boot issue on some ARM boxes with NVMe behind a Synopsys
DesignWare PCIe host controller.

Fixes: 7fd8930f26be4 ("nvme: add a common helper to read Identify Controller data")
Signed-off-by: Ard Biesheuvel <>
Signed-off-by: Sagi Grimberg <>
Signed-off-by: Sasha Levin <>
drivers/nvme/host/pci.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 1ac4cec5f4f7c..e2bce9385eda6 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -1863,7 +1863,7 @@ static int nvme_pci_reg_write32(struct nvme_ctrl *ctrl, u32 off, u32 val)

static int nvme_pci_reg_read64(struct nvme_ctrl *ctrl, u32 off, u64 *val)
- *val = readq(to_nvme_dev(ctrl)->bar + off);
+ *val = lo_hi_readq(to_nvme_dev(ctrl)->bar + off);
return 0;


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